The present invention relates to a hydraulic control system for displacing a synchronizer into engagement with a rotating gear in a gearbox.
It is known for an engine to drive two wet clutches that lie in parallel torque output paths. The output of the first clutch in the first torque transmission path directly drives the input shaft of a gearbox. A first gear that is fast in rotation with the gearbox input shaft directly meshes with a first gear that is rotatably supported on a lay shaft driven by the output of second clutch in the second torque transmission path. Similarly, a second gear on the shaft meshes by way of a reversing idler gear with a second gear that is rotatably supported on the lay shaft. A synchronizer movable by a selector over the lay shaft can couple the lay shaft selectively for rotation with the first and second gears.
When the tractor is being driven, one or other of the two clutches is engaged. The effect of switching between the two torque transmission paths, by disengaging one clutch at the same time as engaging the other, depends on the position of the synchronizer. If the synchronizer is engaged with the first gear, then the two torque transmission paths drive the gearbox in the same direction but with different transmission ratios, resulting in a change between a higher and a lower ratio. On the other hand, if the synchronizer is engaged with the second gear, then the torque is reversed when transmitted through the second path on account of the idler gear and switching between the two torque transmission paths causes a power shuttle to occur, that is to say the tractor changes between a low forward gear and reverse gear.
Movement of the synchronizer is carried out while the second clutch is disengaged to select the operating mode, that is to say to switch between High/Low Mode and Shuttle Mode. When the second clutch is disengaged, the lay shaft is simply floating and its speed and direction of rotation will be determined by the balance of the frictional forces acting on it. For this reason, the synchronizer will not normally be rotating at the same speed as the gear with which it is about to engage, be it the first or the second gear, and if it is simply forced into rapid engagement by suddenly applying a force to it using a hydraulic piston/cylinder unit, then wear will be caused to the synchronizer and the gears.
The present invention therefore seeks to provide a electro-hydraulic control system for displacing a synchronizer into engagement with a rotating gear in a gearbox, which reduces wear on the mechanical components and permits rapid and smooth engagement of the synchronizer with the gear.